The invention relates to a new class of living free radical initiators that are based on alkylperoxydiarylborane derivatives with the general formula of Rxe2x80x94[Oxe2x80x94Oxe2x80x94Bxe2x80x94xcfx861(xe2x80x94xcfx862)]n. The initiators exhibit living polymerization at ambient temperature to produce white solid vinyl polymers with pre-determined molecular weight and narrow molecular weight distribution. By sequential monomer addition, the initiators also produce block copolymers with controlled copolymer composition and narrow molecular weight distribution.
The control of polymer structure has been an important facet in polymer synthesis, both for academic interests and industrial applications. A living polymerization mechanism provides an optimal means for preparing polymers having well-defined molecular structures, i.e. molecular weight, narrow molecular weight distribution, polymer chain end, as well as for preparing block and star polymers. In the past, the most viable techniques in living polymerization reactions were mediated by anionic, cationic, and recently metathesis initiators [for anionic living polymerization, see Holden, et al, U.S. Pat. No. 3,265,765; for cationic living polymerization, see Kennedy, et al, U.S. Pat. No. 4,946,899; and for metathesis living polymerization, see R. H. Grubbs, et al, Macromolecules, 21, 1961 (1988)]. However, these polymerization processes are very limited to a narrow range of monomers, due to the sensitivity of active sites to functional (polar) groups.
In many respects, free radical polymerization is the opposite of living ionic and metathesis polymerizations since it is compatible with a wide range of functional groups, but offers little or no control over polymer structure. Despite this drawback, free radical polymerization is the preferred industrial choice in the commercial production of vinyl polymers, especially those containing functional groups.
Early attempts to realize a living free radical polymerization involved the concept of reversible termination of the growing polymer chains by iniferters, such as N,N-diethyldithiocarbamate derivatives [Otsu, et.al, J. Macromol.Sci., Chem., A21, 961 (1984); Macromolecules, 19, 287 (1986); Eur. Polym. J., 25, 643 (1989); Turner, et.al, Macromolecules, 23, 1856 (1990)]. However, this strategy suffered from poor control of polymerization reaction and polymer formed having high polydispersity.
The first living radical polymerization was observed in the reactions involving a stable nitroxyl radical, such as 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO), that does not react with monomers but forms a reversible end-capped propagating chain end [see, Moad, et.al, Polymer Bull., 6, 589 (1082); Georges, et.al, Macromolecules, 26, 2987 (1993); Georges, et.al, U.S. Pat. Nos. 5,322,912 and 5,401,804; Hawker, et.al, J. Am. Chem. Soc., 116, 11185 (1994); and Koster, et.al, U.S. Pat. No. 5,627,248]. The formed covalent bonds reduce the overall concentration of free radical chain ends, which leads to a lower occurrence of unwanted termination reactions, such as coupling and disproportionation reactions. For an effective polymerization, the reaction has to be carried out at an elevated temperature ( greater than 100xc2x0 C.). Relatively high energy is needed in the cleavage of the covalence bond, which maintains a sufficient concentration of propagating radicals for monomer insertion. Furthermore, this living radical polymerization seems effective only with styrenic monomers.
Subsequently, several research groups have replaced the stable nitroxyl radical with transition metal species as the capping agents to obtain a variety of copper, nickel, iron, cobalt, or ruthenium-mediated living free radical systems, so-called atom transfer radical polymerization (ATRP) [see, Matyjaszewski, et.al, Macromolecules, 28, 7901 (1995); J. Am. Chem. Soc., 117, 5614 (1995); Mardare, et.al, U.S. Pat. No. 5,312,871; Sawamoto, et.al, Macromolecules, 28, 1721 (1995); Percec, et.al, Macromolecules, 28, 7970 (1995); Teyssie, et.al, Macromolecules, 29, 8576 (1996); and Fryd, et.al, U.S. Pat. No. 5,708,102]. Overall, all of these systems have a central theme, i.e., reversible termination via equilibrium between active and dormant chain end at an elevated temperature, which is regulated by a redox reaction involving metal ions. The main advantage of this reaction is that, through a proper choice of the metal compound, it is possible to operate with a broad spectrum of monomers. However, a major drawback is the formation of a deep colored reaction mixture that requires extensive purification procedures to obtain the desired final product.
It has also been known that trialkyborane in an oxidized state becomes an initiator for the polymerization of a number of vinyl monomers [see Furukawa, et al, J. Polymer Sci., 26, 234, 1957; J. Polymer Sci. 28, 227, 1958; Makromol. Chem., 40, 13, 1961; Welch, et.al, J. Polymer Sci. 61, 243, 1962 and Lo Monaco, et. al. U.S. Pat. No. 3,476,727]. The polymerization mechanism involves free radical addition reactions. The initiating radicals may be formed from homolysis of peroxyborane or by the redox reaction of the peroxyborane with unoxidized trialkylborane. A major advantage of borane initiators is the ability to initiate the polymerization at low temperature. Peroxides and azo initiators, when used alone, usually require considerable heat input to decompose and thereby to generate free radicals. Elevation of the temperature often causes significant reduction in molecular weight of the polymer accompanied by the loss of important properties of the polymer.
U.S. Pat. No. 3,141,862 discloses conducting a trialkylborane-initiated free radical polymerization in the presence of an alpha-olefin hydrocarbon polymer. Apparently, the graft-onto reaction by this route was very difficult. The inert nature and insolubility of polyolefin (due to crystallinity) also seems to have hindered the process and resulted in very poor graft efficiency. The reactions shown in the examples of this patent also seem to require a very high concentration of organoborane initiator and monomers and to require elevated temperature. The majority products are homopolymers or insoluble gel. No information about the molecular structure of copolymers is provided in this patent.
Despite the advantage of borane initiators, organoborane-initiated polymerizations tend to be unduly sensitive to the concentration of oxygen in the polymerization system. Too little or too much oxygen results in little or no polymerization. High oxygen concentration causes organoborane to be transfered rapidly to borinates, boronates and borates that are poor initiators at low temperature. Moreover, polymerization is often inhibited by oxygen. To facilitate the formation of free radicals, some borane-containing oligomers and polymers [see Bollinger, et.al. U.S. Pat. No. 4,167,616 and Ritter, et.al. U.S. Pat. No. 4,638,092] were used as initiators in the free radical polymerizations. These organoboranes are prepared by the hydroboration of diene monomerss or polymers or copolymers. Similar polymeric organoborane adducts, prepared by the hydroboration of 1,4-polybutadiene and 9-borabicyclo(3,3,1)-nonane (9-BBN), have also been reported in Macromol. Chem., 178, 2837, (1977).
In the past decade, we have been focussing on the selective oxidation of trialkylborane and studying the mono-oxidative adducts as a new free radical initiation system. The research objective was centered around the functionalization of polyolefins by first incorporating borane groups into a polymer chain, which was then selectively oxidized by oxygen to form the mono-oxidized borane moieties that initiate free radical graft-from polymerization at ambient temperature to form polyolefin graft and block copolymers [Chung, et.al, U.S. Pat. Nos. 5,286,800 and 5,401,805; Macromolecules, 26, 3467 (1993); Polymer, 38, 1495 (1997); Macromolecules, 31, 5943(1998); J. Am. Chem. Soc., 121, 6763 (1999); Macromolecules, 32, 8689(1999)]. Overall, the reaction process resembles a transformation reaction from transition metal (metallocene) coordination polymerization to free radical polymerization via the incorporated organoborane groups. Several years ago, a relatively stable radical initiator was discovered, i.e., the oxidation adducts of alkyl-9-borabicyclononane (alkyl-9-BBN) [Chung, et.al, J. Am. Chem. Soc., 118, 705(1996)]. This initiator exhibits the radical polymerization of methacrylate monomers with a linear relationship between polymer molecular weight and monomer conversion in the range of low ( less than 15%) monomer conversion. The polymers formed during the polymerization show a stable but relatively broad molecular weight distribution (Mw/Mn=xcx9c2.5), compared to polymers prepared by living polymerization processes. This initiator is also incapable of producing block copolymers by sequential monomer addition, indicating limited stability at the propagating chain end.
It is an objective of the present invention is to provide a new class of living free radical initiators, based on alkylperoxydiarylboranes, which are pure mono-oxidized borane compounds and can initiate living polymerization at ambient temperature to produce vinyl polymers having a white solid form without the need for performing any purification procedures. The general formula of the alkylperoxydiarylborane derivatives is illustrated below:
Rxe2x80x94[Oxe2x80x94Oxe2x80x94Bxe2x80x94xcfx861(xe2x80x94xcfx862)]n, 
wherein n is from 1 to 4, preferably n is 1 or 2; R is a hydrogen or a linear, branched or cyclic alkyl radical having a molecular weight from 1 to about 500, and xcfx861 and xcfx862, independently, are selected from aryl radicals, based on phenyl or substituted phenyl groups, with the proviso that xcfx861 and xcfx862 can be the chemically bridged to each other with a linking group or with a direct chemical bond between the two aryl groups to form a cyclic ring structure that includes a boron atom. The alkylperoxydiarylboranes may be prepared, for example, by (i) the selective oxidation of alkyldiarylborane by oxygen and by (ii) the condensation reaction between halodiarylborane and alkylhydroperoxide (or the corresponding alkali metal salt).
It is another objective of this invention is to provide a process for polymerizing vinyl monomers by a xe2x80x9clivingxe2x80x9d free radical polymerization process to prepare vinyl homopolymers and copolymers, including block and star-shape copolymers, having well-defined molecular structures, i.e., pre-determined molecular weight and narrow molecular weight distribution. The process involves contacting a mixture comprising one or more free radical polymerizable monomers with an alkylperoxydiarylborane initiator of the present invention at ambient temperature.
The free radical polymerizable monomers contemplated for use in this invention include, for example, methyl methacrylate, ethyl methacrylate, butyl methacrylate, octyl methacrylate, methacrylic acid, methyl acrylate, ethyl acrylate, butyl acrylate, octyl acrylate, 2-hydroxyethyl acrylate, glycidyl acrylate, acrylic acid, maleic anhydride, vinyl acetate, acrylonitrile, acrylamide, vinyl chloride, vinyl fluoride, vinylidene difluoride, 1-fluoro-1-chloro-ethylene, chlorotrifluoroethylene, trifluoroethylene, tertrafluoroethylene, hexafluoropropene, styrene, alpha-methyl styrene, trimethoxyvinylsilane, triethoxyvinylsilane and the like. The radical polymerizable monomers may be used either singly, or as a combination of two or more different monomers.
In the polymerization at ambient temperature, the initiator (Rxe2x80x94[Oxe2x80x94Oxe2x80x94Bxe2x80x94xcfx861(xe2x80x94xcfx862)]n) spontaneously homolyzes at the peroxide bond to form an alkoxyl radical (Rxe2x80x94[O*]n) and an diarylborinate radical (*Oxe2x80x94Bxe2x80x94xcfx861(xe2x80x94xcfx862)). The alkoxyl radical is active in initiating polymerization of the vinyl monomers. On the other hand, the diarylborinate radical is too stable to initiate polymerization due to the back-donating of electron density to the empty p-orbital of boron. However, this xe2x80x9cdormantxe2x80x9d borinate radical may form a reversible bond with the alkoxyl radical at the growing polymer chain end to prevent unwanted termination reactions. This xe2x80x9clivingxe2x80x9d radical polymerization is characterized by a linear increase of polymer molecular weight with monomer conversion, and by a narrow molecular weight distribution (Mw/Mn less than 2.0, typically less than 1.5, preferably less than 1.2), as well as the production of block copolymers by sequential monomer addition.